If two jews only have a civil marriage and divorce do they need a Get?

This is tricky. According to most authorities they do not. Some (a small minority)require it. It may depend on the locale, that is where the woman would want to remarry, but in most locales and circumstances a get would not be required for the woman to remarry and her children of the second union to be considered kosher (non mamzerim). I suggest in all such issues consulting a local dayan (expert in these laws).